1. Field of the Invention
The present invention relates to a positive electrode active material and a lithium secondary battery. More particularly, the present invention relates to a positive electrode active material allowing production of a lithium secondary battery superior in high-temperature cycle characteristics as well as in rate characteristics, and a lithium secondary battery superior in high-temperature cycle characteristics as well as in rate characteristics.
2. Description of Related Art
In recent years, portable electronic devices such as mobile phone, VTR, laptop PC and the like have become smaller and lighter at an accelerated pace. Lithium secondary battery is in use as an electric source of such devices. In general, lithium secondary battery has a high energy density and a high unit-cell voltage of about 4 V; therefore, it is being used not only as an electric source of portable electronic devices but also as an motor driven electric source of electric vehicle or hybrid electric vehicle.
As the positive electrode active material of lithium secondary battery, there are known lithium cobaltate of layered rock salt structure, lithium nickelate of layered rock salt structure, lithium manganate of spinel structure, etc. Lithium cobaltate of layered rock salt structure is unstable in supply because the reserve of cobalt is small and the cobalt-producing regions are unevenly distributed. Also, lithium nickelate of layered rock salt structure has a problem of unstable structure in charging condition.
Lithium manganate of spinel structure, as compared with lithium cobaltate of layered rock salt structure and lithium nickelate of layered rock salt structure, is known to be high in safety as well as rate characteristics and low in cost. The positive electrode active material using lithium manganate of spinel structure has a large internal resistance and it has been attempted to generally add conductive fine particles (e.g. acetylene black) for improvement of electrical conductivity. However, the addition of acetylene black reduces the filling amount of positive electrode active material, which has caused battery capacity decline, in some cases.
For this problem, there was disclosed, in order to reduce the resistance of positive electrode active material per se and provide a lithium secondary battery of high output and high capacity, a lithium secondary battery characterized by using a positive electrode active material which is composed mainly of Li and Mn, which has a cubic spinel structure, and whose primary particles have a nearly octahedral shape constituted mainly by flat crystal faces (see, for example, JP-A-2000-113889). In the lithium secondary battery disclosed in the JP-A-2000-113889, since the positive electrode active material is highly crystalline, the internal resistance is small and large current discharge is possible.